1. Field of the Invention
The invention relates to a LINC transmitter and, in particular, to a multilevel LINC transmitter.
2. Description of the Related Art
To prolong battery life of mobile handset devices, demands on power efficiency of wireless mobile communication systems have become more important. In general, the most power hungry device in a transceiver is a power amplifier which has nonlinear characteristics. In addition, modulation of non-constant-envelope signals demands high linearity of a power amplifier. As a result, a trade off between linearity and power efficiency in a wireless transmitter is necessary.
Various PA linearization techniques have been adopted to improve linearity and power efficiency of wireless transmitters. Linear amplification with nonlinear components (LINC) is a transmitter architecture which increases linearity and power efficiency of a wireless transmitter. Due to accurate signal processing and insensitivity to process variation, a digital LINC architecture is more suitable for modern process technologies.
FIG. 1 is a block diagram of a conventional LINC architecture. As shown in FIG. 1, an input signal S(t) of the LINC 100 is a varying envelope signal. A signal separator 110 receives and divides the input signal S(t) into two constant-envelope signals S1 and S2. Subsequently, two power amplifiers PA1 and PA2 respectively amplify the constant-envelope signals S1 and S2. Since a nonlinear power amplifier can amplify a constant-envelope signal linearly, two power efficient nonlinear power amplifiers are used in such architecture. Finally, the two amplified signals are combined by a power combiner 120. Thus, a linearly amplified signal is obtained at an output of the power combiner 120.
The input of the LINC system is a varying-envelope signal S(t),S(t)=A(t)·ejφ(t) wherein A(t) denotes the signal envelope and φ(t) is signal phase. In the phasor diagram shown in FIG. 2A, the varying-envelope signal S(t) is split into a set of constant-envelope signals, S1(t) and S2(t),
                                             S            ⁡                          (              t              )                                =                    ⁢                                    1              2                        ⁡                          [                                                                    S                    1                                    ⁡                                      (                    t                    )                                                  +                                                      S                    2                                    ⁡                                      (                    t                    )                                                              ]                                                                    =                    ⁢                                    1              2                        ⁢                                          r                0                            ⁡                              [                                                      ⅇ                                          j                      ⁡                                              (                                                                              φ                            ⁡                                                          (                              t                              )                                                                                +                                                      θ                            ⁡                                                          (                              t                              )                                                                                                      )                                                                              +                                      ⅇ                                          j                      ⁡                                              (                                                                              φ                            ⁡                                                          (                              t                              )                                                                                -                                                      θ                            ⁡                                                          (                              t                              )                                                                                                      )                                                                                            ]                                                        And an out-phasing angle θ(t) is expressed as
      θ    ⁡          (      t      )        =            cos              -        1              ⁡          (                        A          ⁡                      (            t            )                                    r          0                    )      Both S1(t) and S2(t) are on a circle with a radius r0. In a conventional LINC transmitter, r0 is a constant scale factor predefined by a system designer. Because input range of an inverse cosine function is [−1, 1], selection of r0 needs to satisfy the formula:r0≧max(A(t))
FIG. 2B illustrates the signals after amplification. The amplified signals are expressed as G·S1(t) and G·S2(t), where G is voltage gain of the power amplifiers. The two amplified signals are combined by a power combiner to obtain a signal √{square root over (2)}G·S(t) which is a linear amplification of the input signal S(t). Because of the out-phasing technique, LINC achieves linear amplification with two power efficient nonlinear power amplifiers.